The present invention relates to a lubricating oil for refrigerator which is used for e.g. a compression type refrigerator such as an air conditioning system or an electric refrigerator. Particularly, it relates to a lubricating oil for refrigerator comprising a polyether which is excellent in compatibility with a hydrofluorocarbon type refrigerant (hereinafter referred to as a HFC type refrigerant) such as 1,1,1,2-tetrafluoroethane (hereinafter referred to as HFC-134a) difluoromethane (hereinafter referred to as HFC-32) or pentafluoroethane (hereinafter referred to as HFC-125).
Heretofore, dichlorodifluoromethane (hereinafter referred to as CFC-12) has been mainly used as a refrigerant for a refrigerator for car air conditioning or cold storage, and chlorodifluoromethane (hereinafter referred to as HCFC-22) has been mainly used as a refrigerant for a refrigerator for room air conditioning. However, with a view to protecting the ozone layer, a HFC type refrigerant such as HFC-134a, HFC-32 or HFC-125, has been developed as a refrigerant which can be substituted for a chlorine-containing refrigerant such as CFC-12 or HCFC-22.
As compared with CFC-12 or CFC-22, the HFC type refrigerant has a high polarity and shows no substantial compatibility with a lubricating oil such as an alkylbenzene or a naphthene type mineral oil which has been commonly used as a refrigerating machine oil. Thus, it tends to lead to double layer separation within the practical temperature range of the refrigerator. Once double layer separation takes place, oil return tends to be poor, which cases various serious problems such that the lubricating oil deposits on the inner wall of an heat exchanger, whereby the heat exchange rate tends to be poor, or lubrication failure or foaming at the start up occurs. Therefore, conventional refrigerating machine oils can not be used as refrigerating machine oils in such a new refrigerant atmosphere.
As refrigerating machine oils which can be used together with a HFC type refrigerant such as HFC-134a, polyether type oils (e.g. JP-B-4-78674, JP-A-1-259095, JP-A-2-102296 and JP-A-3-109492), polyether type oils (JP-A-3-502472, JP-A-3-505602, JP-A-3-88892, JP-3-217494, JP-A-5-25484 and JP-A-5-179268) and carbonate type oils (e.g. JP-A-3-217495, JP-A-4-18490, JP-A-4-63893, JP-A-5-32588, JP-A-5-32992, JP-A-5-186784 and JP-A-6-87791) have been proposed.
These refrigerating machine oils heretofore proposed have a high polarity as compared with an alkylbenzene or a naphthene type mineral oil and accordingly certainly have good compatibility with a HFC-type refrigerant such as HFC-134a. However, as compared with conventional working fluids of CFC-12/mineral oil type and CFC-22/mineral oil type, both the refrigerant and the refrigerating machine oil tend to have a high polarity and tend to contain water.
Accordingly, these refrigerating machine oils have drawbacks that the thermal stability in the co-existence of a HFC type refrigerant such as HFC-134a is poor, and they tend to hydrolyze e.g. a polyethyleneterephthalate film as a coating material of a coil for a motor for compressor. Especially the polyester type oils have a problem that they themselves undergo hydrolysis to form a carboxylic acid, and this carboxylic acid corrode a metal, thus reading to metal erosion. Whereas, the carbonate type oils have a problem that by the hydrolysis, non-condensable carbon dioxide will form.
Further, since a compressor motor is built in the compressor, good electrical insulation is required. However, with the polyether type lubricating oils which are excellent in that they do not undergo hydrolysis, the volume resistivity used to be about 1011 xcexa9/cm, and it can hardly be said that they have good electrical insulation properties.
Whereas, polyether type oils having low hygroscopic properties and good electrical insulation properties, have been proposed in JP-A-4-19295, JP-A-4-39394, JP-4-130188 and JP-A-6-330062. However, such polyether type oils did not fully satisfy all of the properties such as the compatibility with a HFC type refrigerant, low hygroscopic properties and high electrical insulation properties.
It is the object of the present invention to provide a refrigerating machine oil which exhibits good compatibility with a HFC type refrigerant within a wide temperature range and which is excellent in the electrical insulation properties, the lubricating properties, the low hygroscopic properties, durability, etc., and a composition for refrigerator comprising such an oil and a HFC type refrigerant.
The present inventors have paid an attention to a polyoxyalkyleneglycol which has no problem of hydrolysis and have conducted a study earnestly to overcome above mentioned drawbacks of a polyoxyalkylene glycol oil, and they have found that a polyoxyalkylene glycol derivative having a certain structure is excellent in compatibility with a HFC type refrigerant and has a high volume resistivity of at least 1012 xcexa9/cm and have arrived at the present invention.
The present invention is a lubricating oil for refrigerator comprising a polyether represented by the following general formula (1):
R1xe2x88x92[Xxe2x80x94OR4]sxe2x80x83xe2x80x83(1)
wherein R1 is a s-valent group having an aromatic nucleus, X is a bivalent group comprising average q xe2x80x94OR2xe2x80x94 and average r xe2x80x94OR3xe2x80x94, R2 is an ethylene group which may have a methyl group or an ethyl group, R3 is an ethylene group having an alkoxymethyl group wherein the carbon number of the alkoxy group is from 1 to 4, R4 is a C1-10 alkyl group, s is an integer of from 1 to 6, q is from 0 to 6, and r is at least 0.5, provided that (q+r)xc3x97s is a number of from 3 to 80.
R1 of the polyether represented by the general formula (1) in the present invention, is a s-valent group having an aromatic nucleus. s is an integer of from 1 to 6, and s is preferably an integer of from 1 to 2 from the availability of raw material. As a monovalent group having an aromatic nucleus, an aryl group or an aralkyl group, may, for example, be mentioned.
The aryl group is a residue having one hydrogen atom directly bonded to an aromatic nucleus removed from an aromatic hydrocarbon, and it may, for example, be a phenyl group, a biphenyl group or a naphthyl group, or such a group having an alkyl moiety. The aralkyl group is one having a hydrogen atom of an alkyl group substituted by an aryl group, and in other words, it is one having an aryl group bonded to one of the bonds of an alkylene group.
As a 2 to 6 valent group having an aromatic nucleus, a residue having from 1 to 5 hydrogen atoms removed from an aryl group or an aralkyl group, or a group having a hydroxyl group removed from a bisphenol such as bisphenol A, bisphenol F or bisphenol S, may, for example, be mentioned.
The above mentioned from 1 to 5 hydrogen atoms removed from an aryl group or an aralkyl group, may be either hydrogen atoms directly bonded to the aromatic nucleus or hydrogen atoms bonded to an alkyl moiety or an alkylene moiety directly bonded to the aromatic nucleus, or both of them.
Preferred R1 is a monovalent group having an aromatic nucleus, particularly an aryl group, more preferably a phenyl group or a naphthyl group having an alkyl moiety.
The alkyl moiety of the phenyl group or the naphthyl group having an alkyl moiety may be straight chain, branched chain or cyclic. As the phenyl group or the naphthyl group having an alkyl moiety, the following groups are preferred:
A methylphenyl group, an ethylphenyl group, a propylphenyl group, a butylphenyl group, a pentylphenyl group, a hexylphenyl group, a heptylphenyl group, an octylphenyl group, a nonylphenyl group, a decylphenyl group, a dodecylphenyl group, a dimethylphenyl group, a diethylphenyl group, a dipropylphenyl group, a dibutylphenyl group, a dipentylphenyl group, a dihexylphenyl group, a diheptylphenyl group, a dioctylphenyl group, a dinonylphenyl group, a didecylphenyl group, a didodecylphenyl group, a methylnaphthyl group, an ethylnaphthyl group, a dimethylnaphthyl group, a diethylnaphthyl group, etc.
The alkylene moiety of the aralkyl group may be straight chain, branched chain or cyclic. As the aralkyl group, the following groups are preferred:
A benzyl group, a phenylethyl group, a phenylpropyl group, a phenylbutyl group, a phenylpentyl group, a phenylhexyl group, a phenylheptyl group, a phenyloctyl group, a phenylnonyl group, a phenyldecyl group, a phenyldodecyl group, a methylbenzyl group, a naphthylmethyl group, etc.
The carbon number of such an aryl group or an aralkyl group is preferably at most 14 from the viewpoint of the compatibility with a HFC type refrigerant. If it is larger than that, the compatibility with a HFC type refrigerant such as HFC-134a tends to decrease, and phase separation tends to result. The carbon number of a preferred alkyl group is from 6 to 12, and the carbon number of a preferred aralkyl group is from 7 to 12.
X in the general formula (1) is a bivalent group comprising average q xe2x80x94OR2xe2x80x94 and average r xe2x80x94OR3xe2x80x94. The average q means a number obtained by dividing the total number of xe2x80x94OR2xe2x80x94 in one molecule of the polyether represented by the general formula (1) by the number of the s-valent groups. Likewise, the average r means a number obtained by dividing the total number of xe2x80x94OR3xe2x80x94 in one molecule of the polyether represented by the general formula (1) by the number of the s-valent groups. Further, q is from 0 to 6, and r is at least 0.5, provided that (q+r)xc3x97s is a number of from 3 to 80.
In X, xe2x80x94OR2xe2x80x94 and xe2x80x94OR3xe2x80x94 are bonded so that adjacent R1 and R2, R1 and R3, R2 and R3, R3 and R4, and R2 and R4, are all bonded by ether bonds, like e.g. R1xe2x80x94[OR2xe2x80x94OR3xe2x80x94OR4]s or R1xe2x80x94[OR3xe2x80x94OR2xe2x80x94OR4]s. Likewise, in a case where two or more xe2x80x94OR2xe2x80x94 in X are adjacent to one another, such adjacent R2 and R2 are bonded by an ether bond. The same applies to a case where two or more xe2x80x94OR3xe2x80x94 in X are adjacent to one another.
R2 in the general formula (1) is an ethylene group which may have a methyl group or an ethyl group, i.e. an ethylene group, an ethylene group having an methyl group, or an ethylene group having an ethyl group. As the repeating unit xe2x80x94OR2xe2x80x94, an oxyethylene group [xe2x80x94OCH2CH2xe2x80x94], an oxypropylene group [OCH2CH(CH3)xe2x80x94], or an oxy-1,2-butylene group may be mentioned. In one molecule, xe2x80x94OR2xe2x80x94 may be one member selected from an oxyethylene group, an oxypropylene group and an oxy-1,2-butylene group, or may contain two or more members selected therefrom. Here, the oxy-1,2-butylene group means xe2x80x9cxe2x80x94OCH2CH(C2H5)xe2x80x94xe2x80x9d, xe2x80x9cxe2x80x94OCH(CH3)CH(CH3)xe2x80x94xe2x80x9d or xe2x80x9cxe2x80x94OCH2C(CH3)2xe2x80x94xe2x80x9d.
As such a repeating unit xe2x80x94OR2xe2x80x94, one essentially containing an oxypropylene group is preferred from the viewpoint of the viscosity property and the volume resistivity. In the case of a polyether, in order to improve the volume resistivity, it is effective that the content of an oxy-1,2-butylene group is high rather than an oxyethylene group or an oxypropylene group. However, the compatibility with a HFC type refrigerant inversely decreases. Accordingly, the content of an oxy-1,2-butylene group in the total weight of xe2x80x94OR2xe2x80x94 in one molecule, is preferably at most 70wt %.
The oxyethylene group and the oxypropylene group are formed by ring-opening ethylene oxide and propylene oxide, respectively. The oxy-1,2-butylene group is formed by ring-opening 1,2-buthylene oxide, 2,3-butylene oxide or isobutylene oxide. Oxy-1,2-butylene groups in one molecule may be those obtained solely from the above mentioned same butylene oxide or may be oxyalkylene groups obtainable from the above mentioned two or more butylene oxides.
R3 in the general formula (1) is an ethylene group having an alkoxymethyl group wherein the carbon number of the alkoxy group is from 1 to 4. As the repeating unit xe2x80x94OR3xe2x80x94, an oxy(methoxymethyl )ethylene group, an oxy(ethoxymethyl)ethylene group, an oxy(propoxymethyl)ethylene group and an oxy(buthoxymethyl)ethylene group may be mentioned, and it may be one member selected from these or may be two or more members selected from these. In the case of two or more members, their linkage may be block or random. Here, xe2x80x94OR3xe2x80x94 may be represented by the general formula [xe2x80x94OCH2CH(CH2OCnH2n+1)xe2x80x94, wherein n is an integer of from 1 to 4].
As the repeating unit xe2x80x94OR3xe2x80x94, one containing an oxy(methoxymethyl)ethylene group or an oxy(ethoxymethyl)ethylene group, is preferred from the viewpoint of the compatibility with a HFC type refrigerant and the viscosity characteristic. Further, in order to improve the volume resistivity, it is effective that the content of an oxy(butoxymethyl)ethylene group is high rather than the content of an oxy(methoxymethyl)ethylene group. However, the compatibility with a HFC type refrigerant inversely decreases. Accordingly, the content of an oxy(butoxymethyl)ethylene group in the total weight of xe2x80x94OR3xe2x80x94 in one molecule, is preferably at most 70 wt %.
As the oxy(alkoxymethyl)ethylene group, one formed by ring-opening an alkylglycidyl ether such as methylglycidyl ether, ethylglycidyl ether, propylglycidyl ether or butylglycidyl ether, may be mentioned. Oxy(alkoxymethyl) ethylene groups in one molecule may be those obtainable solely from the above mentioned same alkyl glycidyl ether, or may be oxy(alkoxymethyl) ethylene groups obtainable from the above-mentioned two or more alkylglycidyl ethers. In the case of two or more members, their linkage may be block or random.
In X in the general formula (1), linkage of xe2x80x94OR2xe2x80x94 and xe2x80x94OR3xe2x80x94 may be block or random.
R4 in the general formula (1) is a C1-10 alkyl group. The C1-10 alkyl group may be straight chain, branched chain or cyclic. Specific examples of the alkyl group include a methyl group, an ethyl group, a n-propyl group, an isopropyl group, a butyl group, a pentyl group, a hexyl group, a heptyl group, an octyl group, a nonyl group, a decyl group, a cyclopentyl group and a cyclohexyl group. From the viewpoint of the compatibility with a HFC type refrigerant, R4 is preferably a C1-4 alkyl group.
In the general formula (1), s is an integer of from 1 to 6, and when s is 2 or more, the plurality of X in one molecule may be the same or different. q is from 0 to 6, and r is at least 0.5, provided that (q+r)xc3x97s is a number of from 3 to 80. q being 0 means that there is no xe2x80x94OR2xe2x80x94 in one molecule of the polyether represented by the general formula (1).
If the value of (q+r)xc3x97s exceeds 80, the object of the present invention can not adequately be accomplished from the viewpoint of the kinematic viscosity. A preferred value of (q+r)xc3x97s is from 3 to 40, and a more preferred value of (q+r)xc3x97s is from 3 to 15. From the viewpoint of the solubility, preferred q is from 0 to 5, and more preferred q is from 1 to 4. From the viewpoint of the kinematic viscosity, preferred r is from 0.5 to 10, and more preferred r is from 1 to 7.
The polyether represented by the general formula (1) to be used in the present invention may be synthesized by various methods. For example, the following method may be mentioned when the polyether has xe2x80x94OR2xe2x80x94.
A method wherein an alkylene oxide and an alkylglycidyl ether are added to a compound represented by the general formula R1(OH)s (wherein R1 is a s-valent group having an aromatic nucleus, and s is an integer of from 1 to 6) to obtain a polyether, which is then alcoholated with alkali and then reacted with an alkyl monohalide for etherification. As the alkyl monohalide, a C1-10 alkyl chloride, bromide or iodide may, for example, be mentioned.
By simultaneously carrying out addition of the alkylene oxide and the alkylgrycidyl ether to the compound represented by the above general formula R1(OH)s, one wherein linkage of xe2x80x94OR2xe2x80x94 and xe2x80x94OR3xe2x80x94 is random, can be obtained, and by separately carrying out addition of the alkylene oxide and the alkylgrycidyl ether, one wherein the linkage of xe2x80x94OR2xe2x80x94 and xe2x80x94OR3xe2x80x94 is block, can be obtained.
The kinematic viscosity at 100xc2x0 C. of the polyether in the present invention, is preferably from 1 to 100 cSt, more preferably from 2 to 30 cSt. Further, the kinematic viscosity at 40xc2x0 C. is preferably from 1 to 10,000 cSt, more preferably from 5 to 1,000 cSt. Particularly preferred kinematic viscosity at 40xc2x0 C. is from 5 to 500 cSt. Further, the double layer separation temperature at a low temperature with a HFC type refrigerant is preferably not higher than 0xc2x0 C., more preferably not higher than xe2x88x9210xc2x0 C.
To the lubricating oil for refrigerator of the present invention, a mineral oil, a polyxcex1-olefin oil, an alkylbenzene oil, a polyolester oil, a polyether oil other than the polyether oil of the present invention, a perfluoropolyether oil, a fluorinated silicone oil, a phosphate oil, or a tartarate oil may be incorporated within a range not to impair the compatibility with a HFC type refrigerant. The blend ratio is preferably from 1 to 300 parts by weight, more preferably from 10 to 100 parts by weight, per 100 parts by weight of the lubricating oil for refrigerator of the present invention.
The present invention further provides a composition for refrigerator comprising a HFC type refrigerant and the above mentioned lubricating oil for refrigerator. The proportions of the lubricating oil and the refrigerant in the composition for refrigerator of the present invention, are not particularly limited.
To the lubricating oil of the present invention, various additives which have been commonly incorporated to oil agents, such as a withstand load additive, an acid binding agent, an antioxidant, a metal inactivating agent, an extreme pressure agent, a viscosity index improver, etc., may be added as the case requires. As the above acid binding agent, it is effective to add, for example, a compound having an epoxy group, an orthoester, acetal or carbodiimide. Further, it is effective to add an abrasion preventive agent such as a triaryl phosphate, a triaryl phosphite, a trialkyl phosphate, a trialkyl phosphite, zinc alkyldithiophosphate, zinc arylthiophosphate or molybdenum dithiocarbamate, or a viscosity index improver such as grycerol ether. Further, in order to improve the thermal stability, a phenol type compound or an aromatic amine type compound having an radical-binding ability may be added.
For the composition for refrigerator of the present invention, various HFC type refrigerants may be used alone or in admixture. The HFC type refrigerant to be used in the present invention, may be one member or a mixture of two or more members selected from HFC-34, HFC-125, HFC-134a, 1,1,1-trifluoroethane (hereinafter referred to as HFC-143a), 1,1-difluoroethane and 1,1,2,2-tetrafluoroethane.
As the mixture, particularly preferred is a mixture of HFC-32, HFC-125 and HFC-135a, a mixture of HFC-125, HFC-134a and HFC-143a, a mixture of HFC-125 and HFC-143a, or a mixture of HFC-32 and HFC-125.